The present invention concerns an implant for replacement of one or more vertebral bodies and their adjacent disks. More particularly, the vertebral body replacement is particularly well suited for implantation through an anterior approach.
The treatment of injuries to the spine has advanced significantly since the days of the first recorded surgical procedure for spinal cord injury in the late 7th Century. The techniques, instrumentation and implants have changed over the years and have been better adapted to address many forms of spinal injury and deformities that can occur due to trauma, disease or congenital effects. One type of spinal deformity, a kyphosis, involves a prolapse of the vertebral column towards the front of the body, often caused by the destruction of the vertebral body itself. This destruction can be in the form of a trauma type injury, such as a fracture or burst injury to the vertebral body, or a non-traumatic deformity caused by a tumor or a degeneration of the bone in the vertebral body.
Treatment of a kyphosis in the thoracic or lumbar spine appears now to be best achieved through an anterior approach, particularly in order to avoid some of the more severe complications associated with support or replacement of a damaged vertebral body. In most treatments of a kyphosis, a high degree of anterior reconstruction of the spine is required, most frequently involving total removal of the damaged vertebral body. In a typical anterior approach, partial or total ablation of the vertebral body and the two adjacent vertebral disks is carried out. The remaining space is then distracted to manipulate the spine to its correct orientation.
In many cases, the space is filled with a polymerizable paste or a bone graft which is frequently modeled to give it the shape of the destroyed vertebral body. Frequently, autologous bone, such as that extracted from the ilium, is used to bridge the space. The polymerizable paste can include a PMMA bone cement. Once the cavity remaining after the removal of the original vertebral body has been filled, an osteosynthesis instrument is positioned between the adjacent unaffected vertebrae to prevent any relative movement therebetween. The osteosynthesis device is essential to restabilize the vertebral column, to support the loads to which the thoracic or lumbar spine is exposed, and to enhance the likelihood and quickness of union of the bone graft material with the adjacent vertebral bodies. Once the bone graft and material is sufficiently solid, the osteosynthesis device normally is not subjected to any further mechanical stresses.
A known osteosynthesis device is depicted in U.S. Pat. No. 5,108,395 to Jean-Marie Laurain, the disclosure of which is incorporated herein by reference. This system is illustrated in FIGS. 1 and 2 of the present application. Referring first to FIG. 1, it can be seen that a damaged vertebra V3 includes a destroyed vertebral body C3. An interior implant 1 is provided for bridging between the two intact vertebrae V2 and V4 to permit removal of the damaged vertebra V3 and its adjacent disks D2 and D3. The anterior implant 1 includes a pair of clamps 2 which are engaged to the intact vertebral bodies by way of a number of spikes 3. In addition, the clamps 2 are maintained in position by bone screws 5 extending through screw holes 11, lateral lugs 8 of the clamps. The implant 1 also includes a plate 6 which is configured to span between the intact vertebrae and is strong enough to support the loads generated in the spinal column at that location.
Each clamp 2 includes a threaded post 12 projecting therefrom which is configured to pass through a corresponding opening 14 at each end of the plate 6. A nut 7 is adapted to engage the threaded post 12 to fix the plate 6 to each of the clamps 2. The surface of the clamps 2 include serrations 15 which mate with corresponding serrations 16 at each end of the plate 6, thereby permitting differing angular orientations of the plate relative to each of the clamps. An opening 9 is provided through the threaded post 12 of the clamps to receive another bone screw 5 for firm fixation of the clamp with the healthy vertebral bodies V2 and V4.
An important feature of the system described in the ""395 patent is the provision of notches 18 in each of the clamps 2. The notches are configured to receive the tips of a forceps 19 which is used to provide a distraction force between the two vertebrae V2 and V4. As shown in FIG. 2, once the clamps 2 are fixed to the corresponding intact vertebrae, the forceps 19 are used to distract and permit room for placement of a bone graft G. Once the bone graft is in place, the anterior plate 6 can be attached to each of the clamps 2 in the manner previously described. Once the plate is in position, the distraction forceps 19 is removed and the nut 7 tightened to form a rigid construct.
The anterior construct shown in the ""395 patent and in FIGS. 1 and 2 of this application is one system for providing anterior fixation with the use of autologous or allogenic bone graft material. Other implants have been devised which rely upon an additional element interposed between the adjacent vertebra, in lieu of or in addition to the traditional bone graft material. One such device is shown in the patent to Harms et al. U.S. Pat. No. 4,820,305, which is sold as the xe2x80x9cHarms Cagexe2x80x9d by the Biedermann-Motech Company. This device contemplates a hollow cylindrical mesh which is inserted in the gap between adjacent vertebra, with bone graft material being disposed inside the hollow interior of the mesh.
The patent to Brantigan, U.S. Pat. No. 5,192,327, shows a device similar to the xe2x80x9cHarms Cagexe2x80x9d which is composed of a number of hollow oval-shaped implants within which bone graft material is disposed. European Patent No. 0 179 695 to Kehr shows a rigid inert body having a number of passageways extending between the intact vertebrae into which bone growth material can be implanted. In addition, the device shown in the Kehr European patent includes a plate spanning between the vertebrae having holes for receiving bone screws therethrough.
Another variety of implant devices particularly suited for replacement of vertebral bodies include components of generally solid construction which completely occupy the empty vertebral space. These devices are represented by the patents to Kapp et al., U.S. Pat. No. 4,554,914; Doty, U.S. Pat. No. 4,599,086; Ogilvie et al., U.S. Pat. No. 4,636,217; and Downey, U.S. Pat. No. 5,147,404. Each of these devices is provided with a spike or similar mechanism for engaging the endplates of the intact vertebrae to maintain the implant in position. A similar construction is followed in the U.S. Pat. No. 5,062,850 to MacMillan et al., although this device includes open space between support columns of the axially fixed vertebral body prosthesis.
In each of the former patents, the implant device requires separate distraction of the intact vertebrae prior to insertion of the device. The following patents show vertebral prosthesis which include some feature for expansion of the device in situ. For example, the Main et al., U.S. Pat. No. 4,932,975, and Barber U.S. Pat. No. 5,236,460 show prostheses that telescope through the admission of a hydraulic fluid. The patents of Rezaian, U.S. Pat. No. 4,401,112; Wu, U.S. Pat. No. 4,553,273 and Daher, U.S. Pat. No. 4,657,550 show devices that expand in situ the manipulation of a threaded component. In addition, the Rezaian patent shows a turnbuckle construct of this type with the addition of a spiked plate engaged in the opposite intact vertebrae to strengthen the construct.
In recent years, the application of anterior approaches to instrumenting the spine has become more prevalent. As these anterior approaches advance, it becomes of greater necessity to provide a vertebral body replacement that meets all of the benefits of anterior surgery without the detriments of the several prior devices. Each of the above-mentioned vertebral body replacements suffer from one or more disadvantages. For instance, some of the devices do not provide means for osteosynthesis between the intact vertebrae. These devices lack features that can either permit bone ingrowth or facilitate placement of bone graft between adjacent healthy vertebrae. It is recognized that a more permanent and stable correction of a kyphotic condition occurs with fusion of a bony mass in place of the replaced vertebra. Thus, any vertebral body replacement should accommodate this aspect. Other vertebral prosthesis offer no means for adjusting the size of the implant to accommodate the specific vertebral anatomy. Further, other of the devices do not contemplate some auxiliary fixation to help provide a stable construct. Each of these needs, and many others, are met by the vertebral body replacement according to the present invention.
The present invention contemplates a vertebral replacement implant and assembly for fixation of the implant in the space left by a removed vertebra between two intact vertebrae. In one aspect, the implant includes a thin-walled cylindrical body sized to occupy a substantial portion of the space between the intact vertebrae. The cylindrical body is hollow with a plurality of apertures through the wall of the body in communication with the interior, to permit bone ingrowth once the implant is implanted. The opposite ends of the cylindrical body carries continuous threads, preferably on the outer surface of the body.
The inventive implant further contemplates a pair of endplates having a surface directed against a corresponding one of the intact vertebrae when the prosthesis is implanted. The endplates each include a cylindrical portion extending from the end surface, which portion includes threads for mating with the threaded ends of the cylindrical body. Preferably, the threads of the endplates are internal to the cylindrical portion. In one aspect, the endplates are themselves hollow to provide communication between the hollow interior of the cylindrical body and the adjacent intact vertebrae. Alternatively, the invention contemplates the addition of an end cap to the implant to close the end surface of the endplates against the adjacent vertebrae in order to provide additional support for weak vertebrae.
In another embodiment, the end cap is formed with a porous material such as porous tantalum provided under the name HEDROCEL(copyright) by Implex Corporation. The end cap may also include a positioning surface to align the end cap in the endplate opening. In another embodiment, the end cap includes an anchor projecting into the hollow interior of the implant to resist dislodgment.
Another inventive aspect resides in the means for fixing the implant to adjacent intact vertebrae. In one embodiment, the means for fixing includes spikes which are integrally attached or threadedly engaged to the endplate. Alternatively, the means for fixing the implant includes blades integrally attached to the endplate.
Another feature of the invention resides in the provision of means for fixing the cylindrical body to each of the endplates to prevent unthreading of the mating threads of the three components of the implant. In one embodiment, the means for fixing includes apertures in the threaded portion of the endplates which are threaded to accept a set screw. Preferably, two set screws are threaded into two such apertures in the endplates to apply a clamping pressure against the cylindrical body engaged with the endplate.
In another embodiment, the means for fixing contemplates a crimpable cylindrical portion of the endplates. In one aspect, the cylindrical portion includes an annular ring, dissected by a crimping notch. The application of a crimping force around the annular ring reduces the notch, and thereby reduces the circumference of the cylindrical portion so it is tightly engaged about the cylindrical body threaded therein.
Another inventive aspect resides in the provision of means for connecting the implant to a longitudinal member extending outside the space left by the removed vertebrae. The longitudinal member may be a plate or a rod that is fixed in a known manner to the adjacent intact vertebrae. Preferably, the longitudinal member can be used to assist in the distraction of the intact vertebrae for insertion of the vertebral replacement implant.
In one embodiment, the means for connecting includes a clamp configured to clamp onto the longitudinal member. The clamp supports a screw directed towards the replacement implant when it is interposed between the intact vertebrae. The cylindrical body of the implant includes a number of apertures threaded to receive the connecting screw. The clamp is preferably slidable along the length of the longitudinal implant to facilitate alignment of the screw with the number of threaded apertures of the cylindrical body. In addition, the clamp includes a spherical seat, and the screw includes a spherical head to permit varying angular orientations of the screw relative to the longitudinal member.
In another embodiment, the means for connecting includes an arm extending from a flange of the endplates. The free end of the arm defines an opening through which the longitudinal member extends. A set screw intersects the opening to provide fixation of the longitudinal member to the arm of the endplates.
One object of the present invention is to provide a vertebral body replacement implant configured to support the space left by removal of a damaged or diseased vertebra. One objective is to provide an implant that can be easily adjusted to vary the overall length of the implant dependent upon the vertebral level into which the implant is interposed. A further objective of the inventive implant is to permit this length adjustment yet provide means for fixing the components to prevent disengagement or unthreading.
A further object is achieved by the present invention by the provision of means for connecting the vertebral replacement implant to a longitudinal member extending along the length of the spine between the adjacent intact vertebrae. The longitudinal member can be used for distracting the space left by the removed vertebra to facilitate insertion of the replacement implant. Yet another object is to provide an implant that can house bone growth material to facilitate fusion of the instrumented level and promote vascularization between adjacent intact vertebrae and the bone growth material in the implant.
One benefit of the vertebral body replacement of the present invention is that it provides a strong implant to support the spinal loads while awaiting fusion of bone growth material between the intact vertebrae. A further benefit is that the implant can be more easily adjusted to accommodate spaces at different vertebral levels. Another benefit is that the implant provides a means for fixation to the existing intact vertebrae when the implant is implanted to prevent disengagement.
Other objects and benefits of the invention can be gleaned from the following written description of the invention, considered together with the accompanying figures and claims.